In use, certain cylinder lock mechanisms of the plate-tumbler type exhibit, at times, problems of difficult key insertion and removal, key plug binding with consequent difficulty in rotating the plug in the barrel, or interference with proper performance of the locking function. These problems arise from the skewing of the key plug in the barrel, caused by radial forces exerted on an end of the plug by, for example, lock accessories attached thereto. The skewing may force the plate tumblers and/or the plug body into frictional contact with internal surfaces of the barrel, or move the tumblers out of alignment with their grooves, to prevent locking.
Skewing is made possible by lateral play or looseness of the plug in the barrel at one end thereof. In lock barrels made by the die casting process, the play is caused by the need to provide a "draft" in bores or internal cavities, for core-pulling purposes. Thus, a draft-induced taper is produced in a barrel bore, whereby the diameter of the bore is greater at one of its ends than at the other. Inasmuch as the plug body is generally cylindrical, it is loose in the bore where the diameter is greater.
The problems caused by play of the plug are more pronounced where the end use necessitates an elongate lock mechanism the elongate barrel of which varies substantially in bore diameter from one end to the other. Manufacturing options available for minimizing the problems are limited, especially where a shoulder must be provided at one end of an elongate barrel. Such a shoulder is provided in the lock mechanism of U.S. Pat. No. 3,418,833, for example.